Structural control of hierarchically-ordered TiO₂ films by water for dye-sensitized solar cells

A facile way of controlling the structure of TiO₂ by changing the amount of water to improve the efficiency of dye-sensitized solar cells (DSSCs) is reported. Hierarchically ordered TiO₂ films with high porosity and good interconnectivity are synthesized in a well-defined morphological confinement arising from a one-step self-assembly of preformed TiO₂ (pre-TiO₂) nanocrystals and a graft copolymer, namely poly(vinyl chloride)-g-poly(oxyethylene methacrylate). The polymer-solvent interactions in solution, which are tuned by the amount of water, are shown to be a decisive factor in determining TiO₂ morphology and device performance. Systematic control of wall and pore size is achieved and enables the bifunctionality of excellent light scattering properties and easy electron transport through the film. These properties are characterized by reflectance spectroscopy, incident photon-to-electron conversion efficiency, and electrochemical impedance spectroscopy analyses. The TiO₂ photoanode that is prepared with a higher water ratio, [pre-TiO₂]:[H ₂O]=1:0.3, shows a larger surface area, greater light scattering, and better electron transport, which result in a high efficiency (7.7-%) DSSC with a solid polymerized ionic liquid. This efficiency is much greater than that of commercially available TiO₂ paste (4.0-%). Dye-sensitized solar cells: Crack-free, hierarchically-ordered TiO₂ films with bimodal pores, high porosity, good interconnectivity, and excellent light scattering properties result in high efficiency dye-sensitized solar cells if combined with solid polymerized ionic liquids (7.7-% at 100 mW-cm^-2).